Saturday, March 31, 2012

The pterodactyl that fell down the back of the sofa, part 2: Busted jaws, gutter casts and something quite unique

If you missed the first post, odds are you won’t grab what’s going on here: best head here to catch up before reading on.

We left our last post with something of a conundrum: a reappraisal of the skull pieces of Istiodactylus latidens specimen NHMUK R3877 – the best known of this historically important species - suggests that it had a jaw some 20 per cent shorter than assumed for an entire century (as detailed in Witton 2012). This revises the jaw length from 423 mm to 333 mm, which may not seem like much of a big deal, but has considerable implications for the taxonomy and functional anatomy of I. latidens, along with increasing the morphological disparity of pterosaurs generally: there are no other pterosaurs with skull proportions like this new, short-faced version of I. latidens. Faced with literally changing the face of this pterosaur as we know it, then, there is obvious good reason to question this finding. First and foremost, are there any other specimens of I. latidens that can shed light on its actual jaw length? Not really: the only other I. latidens skull remains also bear incomplete jaws, so they offer no assistance here. With this in mind, how about revisiting the previous length estimate for the I. latidens jaws, Walter Hooley’s calculations performed way back in 1913? Is there any good reason to continue using this estimate in light of our new assessment? (Image, above, shows Hooley's illustrations of the NHMUK R3877 skull pieces, including a long-skulled reconstruction and the otherwise ignored, 'third' skull piece [middle right]. From Hooley 1913.)

Hooley’s calculations The most eye-catching fact of Hooley’s NHMUK R3877 skull length estimate is that it was not based on any skull anatomy at all but, instead, on the presumed depositional conditions and in situ positioning of the skull remains as preserved in the Vectis Formation cliffs. Note the use of the word ‘presumed’ here, too: NHMUK R3877 was recovered from an avalanche, so their original position within that cliff is not known.

Three siltstone boulders contained NHMUK R3877 (image, above, shows Hooley's sketches of the original boulders. Note how he assumes the bones are continuous: this is important later. From Hooley 1913). Each represents a bit of gutter cast, a high-energy sedimentary deposit that occupies long, scoured channels in previously deposited substrates. Such deposits are associated with large-scale, sediment-heavy currents and probably indicate that NHMUK R3877 was washed into the Vectis Formation sedimentary basin (presumed to represent a freshwater-brackish lagoon) by a storm. Along with the skull material, these boulders also held several incomplete limb bones in parallel alignment with the long axes of the cranial remains (see adjacent image). Two of these blocks fitted together perfectly (Hooley 1913), but a fourth – presumed to contain the missing mid-lengths of the limb bones and the majority of the missing skull pieces – was never recovered, so the third could not be reattached to the others. Hence, Hooley had to estimate just how big this missing block was. He presumed that the limb bones spanning the missing region were complete when deposited because their relative positions were identical across the gap, and thought the same must also apply to the skull, seeing the cranial extremities were preserved in the same approximate positions in each block. Hence, if Hooley could figure out the length of the limb bones, he would know both how large the missing block was and be able to deduce how much skull material was missing.

Happily, Hooley had some help in this endeavour: the complete humerus of the holotype of I. latidens (NHMUK R176). This specimen has very similar proportions to that of the NHMUK R3877 humerus, so was probably of similar length – 220 mm. Armed with this data, Hooley reasoned that the missing block was 89 mm long and, with this in mind, concluded that 283 mm of missing material length lay between the rostral and braincase pieces of NHMUK R3877. From this, Hooley suggested that NHMUK R3877 had a skull length of 560 mm, of which 423 mm represented jaw. If true, this would mean that the ‘third’ skull element of NHMUK R3877 discussed in our last post would only represent half the of the missing jaw length. This would not be entirely impossible given how parallel sided the maxillary region of I. latidens is, but…

The skull of NHMUK R3877 was clearly totalled when it was depositedHooley’s assumptions that the limb bones of NHMUK R3877 were continuous when deposited does not seem unreasonable as they show no signs of damage or breaking before deposition. The same cannot be said for the skull. The rostrum was clearly smashed at some point during transportation. Check it out for yourself: here’s the rostrum in right lateral and dorsal view (from Witton 2012).

See those big cracks there? They aren’t sutures between bones or unprepared matrix: they’re big fractures between broken regions of bone. Note how the posterior part of the rostrum has actually been displaced from the anterior (arrows indicate points of displacement): the damage is substantial enough that the rostrum no longer represents the in vivo appearance of this specimen. This may, of course, be expected from a specimen found in a gutter cast: high-energy deposits are unlikely to be kind to fragile cargo, and the long, relatively slender bones of istiodactylid skulls may have be prone to shattering in rough transportation. Indeed, the scours that gutter casts infill need large objects to scour out their grooves in the first place: perhaps the remains of this I. latidens were the tools used to make the scour in this instance*.

*This is a good example, by the way, of why palaeontologists need to know something of sedimentology and taphonomy, even if their primary interest is in the biology of extinct animals. Take note if you’re a palaeontology undergraduate who’s bored silly by sedimentology lectures: you genuinely need to know this stuff!

All considered, we can conclude at least two things:

We cannot assume – as Hooley did – that the proportions of the posterior skull will neatly taper to the preserved rostrum. The rostrum was clearly taller in life than it is in this specimen.

We have good evidence that the skull was badly damaged when deposited, which may mean it wasn’t articulated and continuous when preserved. The association of the upper and lower jaw tips indicates that the damage occurred close to, or during, deposition, or else these elements may have separated when tumbled around during further transportation.

Thus, there’s good taphonomic evidence that Hooley’s assumptions of skull continuity being flawed, which undermines his elongate I. latidens skull reconstruction. Frustratingly, we cannot directly test Hooley’s ideas anymore: we have no record of the specimen in situ, and the original composition of the NHMUK R3877 blocks is no longer clear they’ve been entirely prepped away. With no other hypotheses on the table, it seems that the only reliable indication of I. latidens jaw length can now be achieved with the ‘forgotten’ skull piece, which can be directly measured to give us an idea of the jaw metrics. Happily, as mentioned in the last post, there seems to be good morphological congruence between all three pieces of the NHMUK R3877 skull, so this doesn’t seem an unreasonable move at all, and I think we can be confident that the significant proportions of the I. latidens skull can now be predicted. And, if so, it turns out that they’re quite unusual.

A makeover for Istiodactylus latidensWe mentioned at the top of the post that the jaw length of I. latidens can now be measured at 333 mm (as composited and reconstructed, above. From Witton 2012). As such, we can now estimate the entire skull length at around 450 mm (the posteriormost part of the braincase is poorly known, so this remains an estimate until more remains are found), which is considerably shorter than the 560 mm assumed by Hooley and others. In this configuration, the posterior skull is very large with a height nearly 40 per cent of the jaw length, and a width at the jaw joint around a third of the same dimension. For a ‘long snouted’ pterosaur (that is to say, a species that is not considered characteristically short-faced like anurognathids or tapejarids), these values are huge (well, if we ignore taxa that increase their skull heights with crests. That’s cheating.). The tooth row, already considered to be very short, is now even shorter in occupying only the first 27 per cent of the jaw. In addition, this reconstruction brings the aforementioned slenderness of the maxilla to our attention (to my knowledge, the first time this has really been acknowledged): even with the shorter skull configuration offered here, it’s extremely thin with a height not even 2 per cent of the jaw length. Wow. In short, it seems that familiar-old Istiodactylus latidens, an animal we thought we knew well for many years, may be even more unusual than we've given it credit for.

If this assessment is correct, I. latidens is now one of the most easily characterised pterosaurs out there. This isn’t such a big deal, though: there has never been any doubt over the validity or diagnosis of this species. Far more interest stems from what this new skull reconstruction may mean for the workings of its skull during feeding (seriously, what can you eat with a cheek bone thinner than a pencil in a skull over 400 mm long?) and what all this may mean for istiodactylid taxonomy. We’ll discuss these in the final post in this little series.

References

Hooley, R. W. 1913. On the skeleton of Ornithodesmus latidens; an Ornithosaur from the Wealden Shales of Atherfield (Isle of Wight). Quarterly Journal of the Geological Society, 96, 372-422.

2 comments:

I put off reading this and the preceding post until I had time and attention, so this is quite late, but these are a pair of fabulous posts. They're my favourite kind of science writing: explaining how we (and by "we", I mean folk like you) figure things out. The pics are excellent too. I especially like the one from your paper showing the lateral, dorsal, ventral and 'business end' views of the end of the jaws.

Oh, and I'm one of those who is eagerly awaiting the publication of your book, so news the first draft has been submitted is good news indeed to me.

Off to read the paper now, but I noticed the illo at the end. Gruesome. I like the one at the right which appears to be reacting to the one on the left having impinged on its territory. I like how you get expression into the body language.